Background: Postoperative atrial fibrillation (POAF) is a common complication after cardiac surgery, leading to increased mortality, morbidity, and length of stay. Various predictive indices have been validated to identify patients at risk of developing POAF. This study aimed to compare the discriminative ability of four bedside indices that predict POAF in cardiac surgical patients. Method: A total of 2465 consecutive patients who underwent cardiac surgery at an Australian tertiary hospital between 29 December 2015, and 31 December 2022, were retrospectively analysed. Exclusion criteria included pre-existing atrial fibrillation and transcatheter interventions. The area under the receiver-operating characteristic curve (AUC-ROC) analysis and Hosmer–Lemeshow calibration were performed to compare discriminative abilities. A logistic regression model was constructed to evaluate independent risk factors for developing POAF. Results: Older patients (aged > 69) were more likely to develop POAF compared to younger age groups (p < 0.001). The Atrial Fibrillation Risk Index, CHA2DS2-VASC, HATCH and POAF scores had weak discrimination and demonstrated poor predictive ability in this cohort (AUC-ROC 0.49, 0.49, 0.50, 0.52, respectively). Conclusion: Although previously validated in various cohorts, the four bedside predictive indices demonstrated poor discriminative ability and limited generalizability to our tertiary-hospital cohort.